Quadrupolar Solvents Research Articles

Quadrupolarizability of Liquid Mixtures.

Theoretical expressions for the macroscopic polarizability and quadrupolarizability of a quadrupolar mixture are derived. The theory is demonstrated on the example of a liquid mixture of methane and nitrogen (nonquadrupolar plus quadrupolar component). It turns out that the dielectric permittivity (the "dipole strength" of the liquid) of this mixture changes little with the composition, while the quadrupolar length ("quadrupolar strength") almost triples as the fraction of nitrogen approaches one. A set of such mixtures can be used as standard quadrupolar solvents to study systematically phenomena such as quadrupolar solvatochromism, the effect of the solvent on the rate of a reaction etc.

Switching of binding site from nonpolar to polar ligands toward cationic benzonitrile revealed by infrared spectroscopy.

Noncovalent interactions of aromatic molecules in their various charge states with their surrounding environment are of fundamental importance in chemistry and biology. Herein, we analyze the infrared photodissociation spectra of mass-selected cationic clusters of benzonitrile (BN, cyanobenzene, C6H5CN) with L = Ar, N2, and H2O (W), in the CH and OH stretch range (2950-3800 cm-1) with the aid of density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level to probe the interaction of this fundamental aromatic cation in its 2B1 ground electronic state with nonpolar, quadrupolar, and dipolar solvent molecules. While Ar and N2 prefer π-stacking to the aromatic ring of BN+ strongly supported by dispersion forces, W forms a bifurcated CH⋯O ionic hydrogen bond to two adjacent CH groups stabilized by electrostatic forces. Comparison of the BN+-L dimers with related aromatic clusters reveals the effect of ionization, protonation, and substitution of functional groups on the type and strengths of the competing ligand binding motifs.

Solute-Solvent Interactions and Excited-State Symmetry Breaking: Beyond the Dipole-Dipole and the Hydrogen-Bond Interactions.

Symmetry breaking of the excited state of a linear quadrupolar acceptor-donor-acceptor molecule was investigated using time-resolved infrared spectroscopy in 55 solvents allowing the influence of several solute-solvent interactions to be examined separately. No symmetry breaking was found in nonpolar solvents irrespective of their refractive index, indicating that differences in dispersion interactions between the two arms of the molecule do not suffice to induce an asymmetric distribution of the excitation. However, symmetry breaking was observed in nondipolar but quadrupolar solvents like benzene to an extent that can be as large as that found in medium dipolar solvents like THF. Whereas larger symmetry breaking occurs in the most dipolar solvents, the strongest are observed in protic solvents due to hydrogen bonding. Strong evidence of the formation of halogen bonds in the excited state is also presented, confirming the idea of symmetry-breaking-induced asymmetrical photochemistry.

Structural anomaly and dynamic heterogeneity in cycloether/water binary mixtures: Signatures from composition dependent dynamic fluorescence measurements and computer simulations

We have performed steady state UV-visible absorption and time-resolved fluorescence measurements and computer simulations to explore the cosolvent mole fraction induced changes in structural and dynamical properties of water/dioxane (Diox) and water/tetrahydrofuran (THF) binary mixtures. Diox is a quadrupolar solvent whereas THF is a dipolar one although both are cyclic molecules and represent cycloethers. The focus here is on whether these cycloethers can induce stiffening and transition of water H-bond network structure and, if they do, whether such structural modification differentiates the chemical nature (dipolar or quadrupolar) of the cosolvent molecules. Composition dependent measured fluorescence lifetimes and rotation times of a dissolved dipolar solute (Coumarin 153, C153) suggest cycloether mole-fraction (X(THF)/Diox) induced structural transition for both of these aqueous binary mixtures in the 0.1 ≤ X(THF)/Diox ≤ 0.2 regime with no specific dependence on the chemical nature. Interestingly, absorption measurements reveal stiffening of water H-bond structure in the presence of both the cycloethers at a nearly equal mole-fraction, X(THF)/Diox ∼ 0.05. Measurements near the critical solution temperature or concentration indicate no role for the solution criticality on the anomalous structural changes. Evidences for cycloether aggregation at very dilute concentrations have been found. Simulated radial distribution functions reflect abrupt changes in respective peak heights at those mixture compositions around which fluorescence measurements revealed structural transition. Simulated water coordination numbers (for a dissolved C153) and number of H-bonds also exhibit minima around these cosolvent concentrations. In addition, several dynamic heterogeneity parameters have been simulated for both the mixtures to explore the effects of structural transition and chemical nature of cosolvent on heterogeneous dynamics of these systems. Simulated four-point dynamic susceptibility suggests formation of clusters inducing local heterogeneity in the solution structure.

Microsolvation of the Formanilide Cation (FA+) in a Nonpolar Solvent: Infrared Spectra of FA+–Ln Clusters (L = Ar, N2; n ≤ 8)

Infrared photodissociation (IRPD) spectra of cationic formanilide (N-phenylformamide) clusters, FA(+)-Ln, with L = Ar (n = 1-8) and N2 (n = 1-6), are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges to probe the preferred interaction motifs and the cluster growth. Cold FA(+)-Ln clusters are generated by electron ionization in a supersonic expansion, which generates predominantly the most stable cluster isomers. Size- and isomer-specific νNH frequencies unravel the microsolvation process of FA(+) in a nonpolar (L = Ar) and a quadrupolar (L = N2) solvent. The H-bound FA(+)-L dimer with L binding to the NH proton of the amide group is the most stable isomer, and further ligands are attached to the aromatic ring (π-stacking). Ionization changes the preferred binding motif from π-stacking to H-bonding in FA((+))-L. Quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level confirm the experimentally derived sequential cluster growth and the vibrational and isomer assignments. The calculated FA(+)-L binding energies of D0(H) = 594/1054 cm(-1) for H-bound and D0(π) = 459/604 cm(-1) for π-bound Ar/N2 ligands are consistent with the observed photofragmentation branching ratios. Ionization of FA results from removal of a bonding π-electron delocalized over the phenyl and amide moieties and thus weakens the N-H bond and strengthens the C-O bond.

Quadrupole terms in the Maxwell equations: Born energy, partial molar volume, and entropy of ions

A new equation of state relating the macroscopic quadrupole moment density Q to the gradient of the field ∇E in an isotropic fluid is derived: Q = αQ(∇E - U∇·E/3), where the quadrupolarizability αQ is proportional to the squared molecular quadrupole moment. Using this equation of state, a generalized expression for the Born energy of an ion dissolved in quadrupolar solvent is obtained. It turns out that the potential and the energy of a point charge in a quadrupolar medium are finite. From the obtained Born energy, the partial molar volume and the partial molar entropy of a dissolved ion follow. Both are compared to experimental data for a large number of simple ions in aqueous solutions. From the comparison the value of the quadrupolar length LQ is determined, LQ = (αQ/3ɛ)(1/2) = 1-4 Å. Data for ion transfer from aqueous to polar oil solution are analyzed, which allowed for the determination of the quadrupolarizability of nitrobenzene.

Microsolvation of the acetanilide cation (AA+) in a nonpolar solvent: IR spectra of AA+–Lnclusters (L = He, Ar, N2; n ≤ 10)

Infrared photodissociation (IRPD) spectra of mass-selected cluster ions of acetanilide (N-phenylacetamide), AA(+)-Ln, with the ligands L = He (n = 1-2), Ar (n = 1-7), and N2 (n = 1-10) are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges of AA(+) in its (2)A'' ground electronic state. Cold AA(+)-Ln clusters are generated in an electron impact ion source, which predominantly produces the most stable isomer of a given cluster ion. Systematic vibrational frequency shifts of the N-H stretch fundamentals (νNH) provide detailed information about the sequential microsolvation process of AA(+) in a nonpolar (L = He and Ar) and quadrupolar (L = N2) solvent. In the most stable AA(+)-Ln clusters, the first ligand forms a hydrogen bond (H-bond) with the N-H proton of trans-AA(+) (t-AA(+)), whereas further ligands bind weakly to the aromatic ring (π-stacking). There is no experimental evidence for complexes with the less stable cis-AA(+) isomer. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra. The calculated binding energies of De(H) = 720 and 1227 cm(-1) for H-bonded and De(π) = 585 and 715 cm(-1) for π-bonded Ar and N2 ligands in t-AA(+)-L are consistent with the observed photofragmentation branching ratios of AA(+)-Ln. Comparison between charged and neutral AA((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π-stacking to H-bonding. Electron removal from the HOMO of AA(+) delocalized over both the aromatic ring and the amide group significantly strengthens the C[double bond, length as m-dash]O bond and weakens the N-H bond of the amide group.

Equation-of-state modeling of mixtures with ionic liquids

A non-electrolyte equation-of-state model was used to describe the phase behavior of binary systems containing alkyl-methyimidazolium bis(trifluoromethyl-sulfonyl)imide ionic liquids. A methodology is suggested for modeling this phase behavior by using the Non-Random Hydrogen-Bonding (NRHB) model. According to this methodology, the scaling constants of the ionic liquid are calculated using limited available experimental data on liquid densities and Hansen's solubility parameters, while all electrostatic interactions (polar, hydrogen bonding and ionic) are treated as strong specific interactions. Using the aforementioned methodology, the model is applied to describe the vapor-liquid and the liquid-liquid equilibria in mixtures of ionic liquids with various polar or quadrupolar solvents at low and high pressures. In all cases, one temperature-independent binary interaction parameter was used. Accurate correlations were obtained for the majority of the systems, both, for vapor-liquid and liquid-liquid equilibria.

Molecular Simulations of Outersphere Reorganization Energies in Polar and Quadrupolar Solvents. The Case of Intramolecular Electron and Hole Transfer

Outersphere reorganization energies (lambda) for intramolecular electron and hole transfer are studied in anion- and cation-radical forms of complex organic substrates (p-phenylphenyl-spacer-naphthyl) in polar (water, 1,2-dichloroethane, tetrahydrofuran) and quadrupolar (supercritical CO2) solvents. Structure and charge distributions of solute molecules are obtained at the HF/6-31G(d,p) level. Standard Lennard-Jones parameters for solutes and the nonpolarizable simple site-based models of solvents are used in molecular dynamics (MD) simulations. Calculation of lambda is done by means of the original procedure, which treats electrostatic polarization of a solvent in terms of a usual nonpolarizable MD scheme supplemented by scaling of reorganization energies at the final stage. This approach provides a physically relevant background for separating inertial and inertialless polarization responses by means of a single parameter epsilon(infinity), optical dielectric permittivity of the solvent. Absolute lambda values for hole transfer in 1,2-dichloroethane agree with results of previous computations in terms of the different technique (MD/FRCM, Leontyev, I. V.; et al. Chem. Phys. 2005, 319, 4). Computed lambda values for electron transfer in tetrahydrofuran are larger than the experimental values by ca. 2.5 kcal/mol; for the case of hole transfer in 1,2-dichloroethane the discrepancy is of similar magnitude provided the experimental data are properly corrected. The MD approach gives nonzero lambda values for charge-transfer reaction in supercritical CO2, being able to provide a uniform treatment of nonequilibrium solvation phenomena in both quadrupolar and polar solvents.

Solubility of Water in a Benzene−Cyclohexane Mixture

The solubility of a water molecule in a binary mixture of nonpolar cyclohexane and quadrupolar benzene is studied with the ab initio method. A novel self-consistent reaction field theory that properly accounts for benzene quadrupole moments in the continuum solvent framework is used to describe the solvation effects of the solvent mixture. The free energy of transfer from pure cyclohexane to the mixture solvent is obtained with the neglect of nonelectrostatic contributions. A reasonable agreement with experiments indicates that the theoretical method presented here provides a promising approach to electronic structure calculations in quadrupolar solvents and their mixtures with nonpolar solvents.

Polar Attributes of Supercritical Carbon Dioxide

AbstractFor Abstract see ChemInform Abstract in Full Text.

Equilibrium solvation in quadrupolar solvents

We present a microscopic theory of equilibrium solvation in solvents with zero dipole moment and nonzero quadrupole moment (quadrupolar solvents). The theory is formulated in terms of autocorrelation functions of the quadrupolar polarization (structure factors). It can be therefore applied to an arbitrary dense quadrupolar solvent for which the structure factors are defined. We formulate a simple analytical perturbation treatment for the structure factors. The solute is described by coordinates, radii, and partial charges of constituent atoms. The theory is tested on Monte Carlo simulations of solvation in model quadrupolar solvents. It is also applied to the calculation of the activation barrier of electron transfer reactions in a cleft-shaped donor-bridge-acceptor complex dissolved in benzene with the structure factors of quadrupolar polarization obtained from molecular-dynamics simulations.

Polar Attributes of Supercritical Carbon Dioxide

Supercritical carbon dioxide (scCO(2)) is increasingly promoted as an environmentally benign alternative to conventional organic solvents. The supercritical state bridges the gap between liquid and gaseous states by offering gaslike diffusion rates and liquidlike solvent densities, thereby enabling potential opportunities as a reaction and separation medium in chemical industry. Understanding the solvent behavior of liquid and scCO(2) is of critical importance to enable the design of CO(2)-philic molecular systems and to expand the use of these solvent systems to a wider range of chemical processes. Historically CO(2) was treated as a nonpolar solvent, primarily because of its low dielectric constant and zero molecular dipole moment. CO(2) has also been described as a quadrupolar solvent because of its significant quadrupole moment. Recent studies suggest that, as far as the microscopic solvent behavior of CO(2) is concerned, CO(2) has the potential to act as both a weak Lewis acid and Lewis base. Also, strong theoretical and experimental evidence indicates that CO(2) can participate in conventional or nonconventional hydrogen-bonding interactions. All of these site-specific solute-solvent interactions are important to understand the fundamental nature of CO(2) as a solvent. In this Account, we discuss these polar attributes of CO(2) and their relation to solvation.

A continuum theory of solvation in quadrupolar solvents. I. Formulation

A continuum theory to describe equilibrium and nonequilibrium solvation in polarizable, nondipolar, quadrupolar solvents is developed. By employing the densities of the solvent quadrupole and induced dipole moments as primary field variables, a reaction field theory formulation for quadrupolar solvents is constructed with account of their electronic polarizability. Nonequilibrium solvation aspects are effected via the solvent coordinate description for the quadrupole moment density. It is found that the theory is consistent with the macroscopic Maxwell equations and satisfies the continuity of the electric potential across the cavity boundaries. Solvation stabilization arising from the solvent quadrupoles is captured via novel reaction field factors analogous to those for dipolar solvents. Comparison is made with the dielectric continuum description of the polarizable, dipolar solvents as well as with previous theories of the quadrupolar solvents. Extensions and applications of the current theoretical formulation to study free energetics and dynamics of reactive and spectroscopic processes in the quadrupolar solvents are reported in the following paper [J. Jeon and H. J. Kim, J. Chem. Phys. 119, 8626 (2003)].

A continuum theory of solvation in quadrupolar solvents. II. Solvation free energetics, dynamics, and solvatochromism

The continuum theory of solvation developed in the preceding paper [J. Chem. Phys. 119, 8606 (2003)] is applied to study free energetics and dynamics relevant to charge shift processes in polarizable quadrupolar solvents in the Born–Oppenheimer regime of solvent electronic degrees of freedom. The outer-sphere solvent reorganization free energy and free energy of reaction for electron transfer systems are investigated in both one and two-sphere cavity descriptions. The quadrupolar susceptibility needed in the theory is determined using recent Stokes shift measurements by Reynolds et al. [J. Phys. Chem. 100, 10337 (1996)]. A good accord with measurements and molecular theory estimates of the solvent reorganization free energy for bridged donor–acceptor systems is obtained. The solvatochromic shifts associated with chromophores are examined with a first-order perturbation method. The application of the theory to electronic absorption of betaine in the quadrupolar solvents yields a good agreement with the ET(30) solvent polarity scale. By incorporating the collective motions associated with solvent quadrupoles, the theory is extended to study inertial solvation dynamics. In contrast with dipolar liquids, the solvent frequency in the quadrupolar solvents increases as the multipole character of the solute charge distribution increases. This is in accord with a recent molecular dynamics simulation result for supercritical CO2.

On the validity of dielectric continuum models in application to solvation in molecular solvents

We report Monte Carlo simulations of solvation of a point dipole in dipolar–quadrupolar solvents of varying dipole moment and axial quadrupole. The simulations are carried out to test the prediction of dielectric solvation models of a monotonic increase of the absolute value of the solvation chemical potential |μp| with the solvent dielectric constant ε. Dielectric constants are obtained from pure liquid simulations carried out for each solvent used in solvation simulations. A raising dependence of |μp| on ε, in qualitative agreement with dielectric solvation models, is seen when the solvent dipole moment is varied at constant solvent quadrupole. An increase in the axial quadrupole at constant solvent dipole reduces the dielectric constant at the same time leading to higher |μp| values. The simulations and dielectric models thus give the opposite dependence on the solvent quadrupole for any solvent dipole. We also show that for solvation in dipolar–quadrupolar solvents the saturation limit |μp|→const at ε≫1 predicted by linear response dielectric continuum models actually occurs in the range of nonlinear solvation.

Interaction between Aromatic Amine Cations and Quadrupolar Ligands: Infrared Spectra of Aniline+−(N2)n (n = 1−5) Complexes

Infrared (IR) photodissociation spectra of mass-selected aniline+−(N2)n complexes with n ≤ 5, produced in an electron impact (EI) cluster ion source, are presented in the range of the N−H stretch vibrations of the aniline cation (An+). Analysis of the An+−N2 spectrum is consistent with a H-bound structure in which the N2 ligand forms a nearly linear hydrogen bond to one proton of the NH2 group of An+. No other isomers are observed in the n = 1 spectrum, implying that H-bound An+−N2 corresponds to the global minimum of the dimer potential. This conclusion is supported by ab initio calculations at the UMP2/6-311G(2df,2pd) level, which predict H-bound An+−N2 (De = 1431 cm-1) to be more stable than other local minima. Systematic vibrational frequency shifts in the spectra of the larger An+−(N2)n clusters provide information about the microsolvation process of An+ in a quadrupolar solvent. The first two N2 ligands fill a first subshell by solvating the protons of the amino group (H bonds), whereas further ligands bind to the aromatic ring of An+ (π bonds). Analysis of the photofragmentation branching ratios yields approximate dissociation energies of the H and π bonds of D0(H) ∼ 1100 ± 300 cm-1 and D0(π) ∼ 700 ± 200 cm-1, respectively.

Dynamics of polar solvation in acetonitrile–benzene binary mixtures: Role of dipolar and quadrupolar contributions to solvation

While dynamics of polar solvation have been tabulated for a wide range of pure polar solvents, substantially less is known about the dynamic response of solvent mixtures. Here, results for polar solvation dynamics are presented for the nonassociating mixture of a dipolar solvent, acetonitrile, and a quadrupolar solvent, benzene. The solvation response observed is sensitive to the mixing of the pure solvents, affecting both the inertial and diffusive components of the solvation response function. Addition of acetonitrile to benzene increases the amplitude of the inertial response. At high benzene mole fractions, the diffusive relaxation reveals a slow component attributed to translational diffusion of the acetonitrile.

Excited-State Charge Transfer Dynamics of p-Dimethylaminobenzonitrile in Quadrupolar Solvents

Excited-state intramolecular charge transfer of p-dimethylaminobenzonitrile (DMABN) in benzene, toluene, and dioxane is studied. By combining the recent continuum quadrupolar solvent theory of Jeon and Kim [J. Phys. Chem. A 2000, 104, 9812] and the two-dimensional formulation of the DMABN photoreaction in dipolar solvents by Fonseca et al. [J. Mol. Liq. 1994, 60, 161], the influence of both solvent quadrupole reorganization and solute twist on the reaction free energetics and dynamics is accounted for. The solution-phase reaction paths are investigated with the aid of experimental information on the frequencies associated with solute torsional and collective solvent quadrupole dynamics. The rate constants and transmission coefficients are analyzed using transition state theory with the neglect of dissipative dynamics. Our results are in reasonable agreement with the experimental findings in quadrupolar solvents.

Quadrupolar Solvent Effects on Solvation and Reactivity of Solutes Dissolved in Supercritical CO2

This article presents an overview of our recent studies of solvation and unimolecular reactivity in liquids and supercritical fluids. We review our research on rotational diffusion and photoisomerization kinetics of diphenylpolyenes in polar liquids and supercritical fluids. These results provide a qualitative demonstration of the influence of solvent quadrupole−solute dipole interactions on chemical reactivity in supercritical CO2. The CO2 quadrupole moment both increases solvent−solute friction experienced by a dipolar diphenylpolyene solute molecule and diminishes the activation barrier of the isomerization reaction via interaction with the polar transition state. We also review our recent studies of quadrupolar solvation and excited-state excimer formation kinetics of ADMA (1-[9-anthryl]-3-[4-N,N-dimethylaniline] propane). The ADMA studies have yielded quantitative confirmation of the importance of quadrupolar solvent−dipolar solute interactions in solvation and reactivity in supercritical CO2. The re...